1. Field of the Invention
The present invention relates to a magnetostrictive load sensor for electromagnetically sensing a given load by using a magnetostrictive effect, and also to a movable unit (or a motorized device) including such a magnetostrictive sensor.
2. Description of the Related Art
Load sensors for use in various types of vehicles and crafts including, for example, motorcycles, personal watercrafts, and electric cars, should have as small a size as possible. To meet such a demand, a magnetostrictive sensor has been used extensively as a small load sensor. A magnetostrictive load sensor transforms a variation in a magnetic property of a member on which load is placed into a voltage variation thereby allowing the given load to be sensed based on the voltage variation. Magnetostrictive load sensors are disclosed in PCT International Application Publication No. WO 2004/065812, Japanese Patent Application Laid-Open Publication No. 2003-57128, and PCT International Application Publication No. WO 2007/004472, for example.
PCT International Application Publication No. WO 2004/065812 discloses a magnetostrictive load sensor for a power assisted clutch system. FIG. 21 illustrates a magnetostrictive load sensor 1100 as disclosed in PCT International Application Publication No. WO 2004/065812. The magnetostrictive load sensor 1100 includes a load receiving portion 1120, a coil 1110 and a case 1130.
The load receiving portion 1120 is made of a magnetic material, has a rod shape, and is inserted into a through hole of the coil 1110. The case 1130 is also made of a magnetic material and houses the load receiving portion 1120 and the coil 1110 therein.
One end of the load receiving portion 1120 extends out of an opening that is cut through one end of the case 1130 and contacts with a press member 1155. The other end of the load receiving portion 1120 is secured by the other end of the case 1130.
The surface of the other end of the case 1130 is in contact with the holder 1152. One end of a spring 1153 contacts the back surface of the holder 1152. The other end of the spring 1153 is fixed by a fixing portion 1154. This spring 1153 applies a pre-load of a predetermined magnitude to the load receiving portion 1120 by way of the holder 1152.
The magnetostrictive load sensor 1100, the press member 1155, the holder 1152, the spring 1153 and the fixing portion 1154 are housed in a sensor unit housing 1151. The holder 1152 is arranged so as to rotate in the sensor unit housing 1151.
Also, a wire 1141 is arranged so as to run through the sensor unit housing 1151 and the press member 1155 and be inserted into the holder 1152. The end 1141a of the wire 1141 is fixed on the back surface of the holder 1152.
A sensor portion 1160 including the magnetostrictive load sensor 1100 is built in a clutch lever. By handling the clutch lever, the wire 1141 is pulled to turn the holder 1152 around a shaft Q of rotation. As a result, the load receiving portion 1120 of the magnetostrictive load sensor 1100 is pressed by the press member 1155. Consequently, the values of the load placed on the magnetostrictive load sensor 1100 change. A variation in impedance representing such a variation in the load value is detected by a signal detecting section (not shown).
The load receiving portion 1100 is magnetized by the current flowing through the coil 1110. That is why when a press load is applied to the load receiving portion 1120, a reverse magnetostrictive effect is produced to cause a variation in permeability and change the AC resistances (or impedances) of a circuit including the inductance of the coil 1110. And by obtaining a voltage variation between the two terminals of the coil 1110, caused by that impedance change, by the signal detecting section, the given load can be detected electromagnetically.
FIG. 22 illustrates a magnetostrictive load sensor 1200 as disclosed in Japanese Patent Application Laid-Open Publication No. 2003-57128.
The magnetostrictive load sensor 1200 includes a sensor unit SU that is made up of a coil (not shown), a detection rod 1220 and a case 1230. The sensor unit SU is arranged so as to move vertically in a holder 1250 and is biased upward by a spring 1213.
By supplying current to the coil of the sensor unit SU, the detection rod 1220 is magnetized. When an external load is placed on the detection rod 1220 of the sensor unit SU by way of a press plate 1214, compressive stress is applied to the detection rod 1220. Then, a reverse magnetostrictive effect is produced, causing a variation in the permeability of the detection rod 1220 and in other magnetic properties. That is why by converting such a variation in magnetic properties into a voltage variation and outputting it, the external load placed on the detection rod 1220 can be detected.
FIG. 23 illustrates a magnetostrictive load sensor 1300 as disclosed PCT International Application Publication No. WO 2004/065812.
The magnetostrictive load sensor 1300 includes a coil 1310, a magnetic circuit defining member 1330, a rod member 1320, two load transfer members 1340a and 1340b, and a housing 1350.
The coil 1310 consists of a bobbin 1311 and a conductor 1312. A through hole 1310h runs through the core of the bobbin 1311, around which the conductor 1312 is wound.
The magnetic circuit defining member 1330 consists of a cylindrical first casing member 1331 and a substantially disk-like second casing member 1332. The first and second casing members 1331 and 1332 are made of a magnetic material and function as a magnetic circuit when the magnetostrictive load sensor 1300 operates.
The coil 1310 is inserted into the first casing member 1331 with an annular elastic member 1319 interposed between them. Meanwhile, the second casing member 1332 may be connected to the end of the first casing member 1331 by, for example, press-fitting the second casing member 1332 into the first casing member 1331.
A circular opening 1331h is cut through the center of one end of the first casing member 1331, while another circular opening 1332h is cut through the center of the second casing member 1332. Spacers SP are attached to these openings 1331h and 1332h. 
A columnar rod member 1320 is inserted to run through the through hole 1310h and the two openings 1331h and 1332h. The rod member 1320 is made of a magnetic material, and therefore, is magnetized by the coil 1310 when the magnetostrictive load sensor 1300 operates.
One end 1320a of the rod member 1320 extends out through the opening 1332h, while the other end 1320b thereof extends out through the opening 1331h. The rod member 1320 is supported by the load transfer members 1340a and 1340b. 
The load transfer member 1340a consists of a columnar shaft portion 1341a and a flange portion 1342a. A circular recess 1343a is bored at the center of the flange portion 1342a. Likewise, the load transfer member 1340b also consists of a columnar shaft portion 1341b and a flange portion 1342b. A circular recess 1343b is bored at the center of the flange portion 1342b. The one end 1320a of the rod member 1320 is inserted into, and connected to, the recess 1343a of the load transfer member 1340a. The other end 1320b of the rod member 1320 is inserted into, and connected to, the recess 1343b of the load transfer member 1340b. 
The housing 1350 includes a cylindrical first housing 1351 and a substantially disk-like second housing 1352. The coil 1310, the magnetic circuit defining member 1330, the rod member 1320 and the load transfer members 1340a and 1340b are all housed in the first housing 1351. The first and second housings 1351 and 1352 are joined together with multiple bolts 1359.
Multiple O-rings O1 through O4, which may be made of an elastic resin, for example, are attached to the first and second housings 1351 and 1352. The shaft portion 1341b of the load transfer member 1340b is elastically supported by the O-ring O1. On the other hand, the shaft portion 1341a of the load transfer member 1340a is elastically supported by the O-ring O4.
When the load applied to the load transfer member 1340a is transferred to the one end 1320a of the rod member 1320, compressive stress is placed on the rod member 1320. Then, a reverse magnetostrictive effect is produced to cause a variation in the permeability of the rod member 1320 and change the impedances of the core portion of the sensor including the coil 1310, the magnetic circuit defining member 1330 and the rod member 1320. As a result, induced electromotive force (i.e., voltage) generated in the coil 1310 changes. By measuring this voltage variation sensed by a peripheral circuit, the load applied to the load transfer member 1340a can be detected. Likewise, even when a load is applied to the load transfer member 1340b, that load can be detected in the same way.
These magnetostrictive load sensors 1100, 1200 and 1300 disclosed in PCT International Application Publication No. WO 2004/065812, Japanese Patent Application Laid-Open Publication No. 2003-57128, and PCT International Application Publication No. WO 2007/004472, respectively, have mutually different sensor holding structures. Specifically, the magnetostrictive load sensor 1100 of PCT International Application Publication No. WO 2004/065812 holds the sensor by applying a pre-load to the load receiving portion 1120 with one end of the case 1130 pressed. On the other hand, the magnetostrictive load sensor 1200 of Japanese Patent Application Laid-Open Publication No. 2003-57128 holds the sensor unit SU by pressing the case 1230 of the sensor unit SU against the holder 1250. And in the magnetostrictive load sensor 1300 of PCT International Application Publication No. WO 2007/004472, a portion of the second casing member 1332 is extended outside of the magnetic circuit and is sandwiched between the first and second housings 1351 and 1352, thereby holding the core portion of the sensor including the coil 1310, the rod member 1320 and magnetic circuit defining member 1330.
In the holding structure of the magnetostrictive load sensor 1100 disclosed in PCT International Application Publication No. WO 2004/065812, however, a pre-load is applied to the load receiving portion 1120. That is why if the magnetostrictive load sensor 1100 is exposed to significant vibrations or impact, the magnitude of the pre-load would vary to make the zero-point output not constant.
The holding structure of the magnetostrictive load sensor 1200 disclosed in Japanese Patent Application Laid-Open Publication No. 2003-57128 will be affected less by vibrations or impact than the counterpart of the magnetostrictive load sensor 1100 of PCT International Application Publication No. WO 2004/065812. However, as a load is applied to the case 1230 that forms a magnetic circuit, the zero-point output could fluctuate too.
In the holding structure of the magnetostrictive load sensor 1300 disclosed in PCT International Application Publication No. WO 2007/004472, the core portion of the sensor is held outside of the magnetic circuit, and therefore, is not affected by vibrations or impact so easily. However, in a situation where the first and second casing members 1331 and 1332 are press-fit into each other, if the press fit load is great, then the magnetic properties will deteriorate due to stress. That is why the press fit load cannot be sufficiently increased. Therefore, if the press fit portions shift from each other due to excessive vibrations or impact, the resistance of the magnetic circuit might vary which would affect the output.
FIGS. 24 and 25 schematically illustrate what will happen in the magnetostrictive load sensor 1300 of PCT International Application Publication No. WO 2007/004472 if the press fit portions shift from each other.
As shown in FIG. 24, if the first casing member 1331 shifted to the right, the area of contact between the first and second casing members 1331 and 1332 would decrease (as indicated by the dashed circles P1 in FIG. 24). As a result, the magnetic resistance would increase causing the zero-point output to vary from its intended value. Besides, the position of the first casing member 1331 would easily change after that due to vibrations or impact, thus causing a further fluctuation in the zero-point output. On top of that, the pressure applied by the first casing member 1331 on the elastic member 1319 would decrease (as indicated by the dashed circles P2 in FIG. 24). In that case, the coil 1310 would move easily due to vibrations or impact, thus making the properties inconsistent or causing a disconnection or short in the lead wire (i.e., the conductor 1312 extending from the coil 1310).
Likewise, as shown in FIG. 25, if the second casing member 1332 shifted obliquely, the area of contact between the first and second casing members 1331 and 1332 would also decrease (as indicated by the dashed circle P1 in FIG. 25). As a result, the magnetic resistance would increase causing the zero-point output to vary from its intended value. Besides, the position of the first casing member 1331 would easily change after that due to vibrations or impact, thus causing a further fluctuation in the zero-point output. On top of that, the pressure applied by the first casing member 1331 on the elastic member 1319 would decrease (as indicated by the dashed circle P2 in FIG. 25). In that case, the coil 1310 would move easily due to vibrations or impact, thus making the magnetic properties inconsistent or causing a disconnection or short in the lead wire (i.e., the conductor 1312 extending from the coil 1310). Furthermore, as the first casing member 1331 and the rod member 1320 would make a tight contact with each other either directly or with the spacers SP interposed (as indicated by the dashed circle P3 in FIG. 25), some load loss would be caused. In that case, not only would the electromagnetic properties vary significantly but it also may no longer be possible to accurately sense loading.